Semiconductor laser

ABSTRACT

A semiconductor laser according to the present invention comprises: a substrate; an n-cladding layer disposed on the substrate; an active layer disposed on the n-cladding layer; a p-cladding layer disposed on the active layer and forming a waveguide ridge; and a diffraction grating layer disposed between the active layer and the n-cladding layer or the p-cladding layer and including a phase shift structure in a part of the diffraction grating layer in an optical waveguide direction. The width of the p-cladding layer is increased in a portion corresponding to the phase shift structure of the diffraction grating layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser, and moreparticularly to a distributed feedback semiconductor laser used as alight source for optical fiber communications, etc.

2. Description of the Related Art

In recent years, as broadband communications and publictelecommunication networks using optical fibers have become widely used,there has been an increasing need to transmit a large amount ofinformation at low cost. To meet such a need, it is necessary toincrease the amount of information that can be transmitted per unittime, that is, to increase the information transmission rate. Actually,the transmission rate has been progressively increased from 600 Mbps to2.5 Gbps, to 10 Gbps.

Such an increase in the transmission rate of optical communicationsdevices has led to an expansion in the market for optical communicationsnetworks for use not only in trunk systems but in access systems(offices, homes), requiring that the optical transceivers employhigh-speed, high-efficiency, yet low-cost light emitting/receivingdevices.

Semiconductor lasers externally modulated by an optical modulator havebeen conventionally used as semiconductor lasers for opticalcommunications. However, when the transmitters and receivers areseparated by a relatively small distance, as in access systems, directlymodulated semiconductor lasers may be used, since they have a simpleconfiguration and hence can be produced at low cost.

Waveguide ridge type distributed feedback laser diodes, buriedheterostructure distributed feedback laser diodes, etc. are used asdirectly modulated semiconductor lasers. (A distributed feedbacksemiconductor laser diode is hereinafter referred to as a DFB-LD.)

One known waveguide ridge type semiconductor laser is formed of a GroupIII-V nitride semiconductor material and has a Fabry-Perot ridge stripestructure. This semiconductor laser has a problem in that if the carrierdensity of the active layer is increased to achieve high output power,the density varies in a lateral direction, these causing hole burningand kinking and hence limiting high-power operation.

To prevent this, a semiconductor laser has been disclosed in which: theresonator is divided into two portions by a line perpendicular to thelength direction of the resonator; and the optical confinement factor ofthe active layer under the ridge stripe structure is lower on the frontend face side than on the rear end face side.

In this example, the front end face side includes a p-typeAl_(0.05)Ga_(0.95)N cladding layer, while the rear end face sideincludes a p-type Al_(0.07)Ga_(0.93)N cladding layer. The p-typeAl_(0.05)Ga_(0.95)N cladding layer has a higher refractive index thanthe p-type Al_(0.07)Ga_(0.93)N cladding layer. (See, e.g., paragraphs[00031], [0041], and [0044] and FIGS. 1 to 3 of Japanese PatentLaid-Open No. 2005-302843.)

Another known waveguide ridge type semiconductor laser is also formed ofa Group III-V nitride semiconductor material and has a Fabry-Perot ridgestripe structure. In order to prevent formation of high-order horizontaltransverse modes and occurrence of kinks as well as preventingdegradation in laser characteristics, this semiconductor laser is formedsuch that the ridge portion includes two taper regions each taperinginwardly from a center portion of the resonator to a respective end ofthe resonator in the length direction, that is, the width of each taperregion is reduced as a respective end of the resonator is approached.(See, e.g., paragraphs [00131], [0024]), and [0025] and FIG. 1 ofJapanese Patent Laid-Open No. 2000-357842.)

Further, one known buried heterostructure semiconductor laser device isconstructed such that a Fabry-Perot semiconductor light-emitting elementand a fiber grating form a resonator. This semiconductor laser devicehas a problem in that the distance between the fiber grating and thesemiconductor light-emitting element is large, resulting in increasedrelative intensity noise due to resonance between the fiber grating andthe light reflective film. This makes it difficult to achieve stableRaman amplification. To address this problem, a semiconductor laserdevice has been disclosed in which: an output side reflective filmhaving a low light reflectance (1% or less) is formed on the lightemitting end face; a reflective film having a high reflectance (70% ormore) is formed on the light reflecting end face or the opposite endface; and the mesa stripe portion including an SCH-MQW active layer hasa tapered shape, specifically, the portion of the mesa near the emittingside reflective film has a small width and the portion of the mesa nearthe high reflectance reflective film has a large width. Thisconfiguration allows the semiconductor laser device to generate laserlight including two or more oscillation longitudinal modes. (See, e.g.,paragraphs [0007] to [0009], [0033], and [0034] and FIG. 1 of JapanesePatent Laid-Open No. 2002-299759.)

Further, another known buried heterostructure semiconductor laser devicehas been proposed to increase the coupling efficiency between thesemiconductor light-emitting element and the optical fiber. In order toemit light having a single-peaked pattern at a narrow emission anglewithout degrading operating characteristics such as the thresholdcurrent and slope efficiency, this semiconductor laser device is formedsuch that: the stripe-shaped active layer for generating laser light hascontinuously changing width throughout substantially the entireresonator region; and the width W1 of the active layer at the laserlight emitting front end face and the width W2 of the active layer atthe rear end face on the opposite side satisfy the relation: W2>W1.(See, e.g., paragraphs [0009], [0010], and [0017] and FIG. 2 of JapanesePatent Laid-Open No. 11-220220.)

In all conventional DFB lasers, whether waveguide ridge type or buriedheterostructure type, the photon density distribution within theresonator is not uniform in the axial direction of the resonator or theoptical waveguide direction.

In the case of a semiconductor laser with a diffraction grating having aphase shift structure, for example, the photon density distributionwithin the resonator is such that the photon density gradually increasesfrom both end faces in the axial direction of the resonator toward thephase shift region and then dramatically increases to its maximum valuewithin the phase shift region. In a semiconductor laser with adiffraction grating having no phase shift structure, on the other hand,the photon density may gradually increase from the emitting end face ofthe resonator toward the rear end face and then may dramaticallyincrease at the rear end face portion, depending on the “end facephase”.

Incidentally, the relaxation oscillation frequency fr of a semiconductorlaser, which represents the high speed response characteristics of thelaser, is proportional to the square root of the product of the photondensity S and the optical confinement factor G. In a conventionalDFB-LD, since the optical confinement factor G is constant in the axialdirection of the resonator, the distribution of the relaxationoscillation frequency fr in the axial direction of the resonator issubstantially the same as the photon density distribution. As a result,the response speed within the resonator varies in the axial direction,that is, the resonator has a non-uniform response speed distribution inthe axial direction.

Specifically, the response speed is high in a high photon density regionand low in a low photon density region. Therefore, when a semiconductorlaser having a non-uniform photon density distribution is subjected todirect modulation operation at high speed (e.g., 10 Gbps), the problemof distortion of the modulated light waveform, etc. arises.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems. It is afirst object of the present invention to construct a semiconductor laseradapted such that the relaxation oscillation frequency does not varymuch in the axial direction of the resonator even if the photon densitydistribution within the resonator is not uniform in the axial direction,allowing the semiconductor laser to achieve good high-speed responsecharacteristics.

According to one aspect of the invention, there is provided asemiconductor laser according to the present invention comprises: asemiconductor substrate of a first conductive type; a first claddinglayer of the first conductive type located on the semiconductorsubstrate; an active layer located on the first cladding layer; a secondcladding layer of a second conductive type located on the active layer;and a diffraction grating layer located between the active layer and thefirst or second cladding layer and including a phase shift structurewhich is formed in a part of the diffraction grating layer in an opticalwaveguide direction, wherein an optical confinement factor is reduced incorresponding to the phase shift structure of the diffraction gratinglayer.

Accordingly, in the semiconductor laser according to the presentinvention, the optical confinement factor is reduced in a specificportion corresponding to the phase shift structure of the diffractiongrating layer in such a way that the product of the photon density andthe optical confinement factor in this specific portion is substantiallyequal to that in the other portions. As a result, the relaxationoscillation frequency in this specific portion corresponding to thephase shift structure is also substantially equal to that in the otherportions. This means that the relaxation oscillation frequency does notvary much in the axial direction of the resonator, preventing distortionof the modulated light waveform and allowing the semiconductor laser toachieve good high-speed response characteristics.

According to another aspect of the invention, there is provided asemiconductor laser according to the present invention comprises: asemiconductor substrate of a first conductive type; a first claddinglayer of the first conductive type located on the semiconductorsubstrate; an active layer located on the first cladding layer; a secondcladding layer of a second conductive type located on the active layer;and a diffraction grating layer located between the active layer and thefirst or second cladding layer, wherein a layer having a higherrefractive index than the first or second cladding layer and locatedbetween said first and second cladding layers, is reduced in thethickness at one end having a higher photon density in an opticalwaveguide direction than at the other end.

Accordingly, in the semiconductor laser according to the presentinvention, r educing the thickness of the high refractive index layerresults in a reduction in the optical confinement factor, making itpossible to prevent the relaxation oscillation frequency from varying inthe axial direction of the resonator. Therefore, when the semiconductorlaser is subjected to direct modulation operation at high speed, it ispossible to reduce the distortion of the modulated light waveform due toa change in the relaxation oscillation frequency or due to non-uniformrelaxation oscillation frequency distribution within the resonator.

According to still another aspect of the invention, there is provided asemiconductor laser according to the present invention comprises: asemiconductor substrate of a first conductive type; a first claddinglayer of the first conductive type located on the semiconductorsubstrate; an active layer located on the first cladding layer; a secondcladding layer of a second conductive type located on the active layer;a diffraction grating layer located between the active layer and thefirst or second cladding layer; and an optical waveguide ridge includingthe a second cladding layer, wherein the waveguide ridge is larger inthe width at one end having a higher photon density in an opticalwaveguide direction than at the other end.

Accordingly, in the semiconductor laser according to the presentinvention, the change in the relaxation oscillation frequency within theresonator reduces in the axial direction. Therefore, when thesemiconductor laser is subjected to direct modulation operation at highspeed, it is possible to reduce the distortion of the modulated lightwaveform due to a change in the relaxation oscillation frequency or dueto non-uniform relaxation oscillation frequency distribution within theresonator.

According to yet another aspect of the invention, there is provided asemiconductor laser according to the present invention comprises: asemiconductor substrate of a first conductive type; a first claddinglayer of the first conductive type located on the semiconductorsubstrate; an active layer located on the first cladding layer; a secondcladding layer of a second conductive type located on the active layer;and a diffraction grating layer located between the active layer andsaid first or second cladding layer, wherein the pitch of thediffraction grating layer is varied in an axial direction of theresonator such that the product of the pitch and an equivalentrefractive index is substantially constant in the axial direction, theequivalent refractive index being determined by the refractive index ofeach layer.

Accordingly, in the semiconductor laser according to the presentinvention, a variation in the oscillation wavelength λ reduces, thesemiconductor laser is allowed to emit laser light with a narrowspectral width predominantly including the desired oscillationwavelength.

Other objects and advantages of the invention will become apparent fromthe detailed description given hereinafter. It should be understood,however, that the detailed description and specific embodiments aregiven by way of illustration only since various changes andmodifications within the scope of the invention will become apparent tothose skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a semiconductor laser according to oneembodiment of the present invention.

FIG. 2 is a cross-sectional view of the semiconductor laser taken alongline II-II of FIG. 1.

FIG. 3 is a cross-sectional view of the semiconductor laser taken alongline III-III of FIG. 1.

FIG. 4 is a cross-sectional view of the semiconductor laser taken alongline IV-IV of FIG. 1.

FIG. 5 is a schematic diagram showing cross-sectional dimensions of thediffracting grating of the semiconductor laser according to oneembodiment of the present invention.

FIG. 6 is a schematic diagram illustrating the photon densitydistribution within the resonator of the semiconductor laser of oneembodiment of the present invention, showing the relationship betweenthe position within the resonator and the photon density.

FIG. 7 is a schematic diagram illustrating the optical confinementfactor distribution within the resonator of the semiconductor laser ofone embodiment of the present invention, showing the relationshipbetween the position within the resonator and the optical confinementfactor.

FIG. 8 is a schematic diagram illustrating the relaxation oscillationfrequency distribution within the resonator of the semiconductor laserof one embodiment of the present invention, showing the relationshipbetween the position within the resonator and the relaxation oscillationfrequency.

FIG. 9 is a cross-sectional view of a variation of the semiconductorlaser according to one embodiment of the present invention.

FIG. 10 is a perspective view of a semiconductor laser according to oneembodiment of the present invention.

FIG. 11 is a cross-sectional view of the semiconductor laser taken alongline XI-XI of FIG. 10.

FIG. 12 is a cross-sectional view of the semiconductor laser taken alongline XII-XII of FIG. 10

FIG. 13 is a cross-sectional view of the semiconductor laser taken alongline XIII-XIII of FIG. 10.

FIG. 14 is a cross-sectional view of a variation of the semiconductorlaser according to one embodiment of the present invention.

FIG. 15 is a cross-sectional view of a semiconductor laser according toone embodiment of the present invention.

FIG. 16 is a cross-sectional view of a semiconductor laser according toa fourth embodiment of the present invention.

FIG. 17 is a schematic diagram showing a pitch of a diffraction gratingaccording to one embodiment of the present invention.

FIG. 18 is a schematic diagram showing the relationship between theposition within a resonator and the equivalent refractive indexaccording to one embodiment of the present invention.

FIG. 19 is a schematic diagram showing the diffraction grating pitchdistribution within the resonator according to one embodiment of thepresent invention.

In all figures, the substantially same elements are given the samereference numbers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a perspective view of a semiconductor laser according to oneembodiment of the present invention; FIG. 2 is a cross-sectional view ofthe semiconductor laser taken along line II-II of FIG. 1; FIG. 3 is across-sectional view of the semiconductor laser taken along line III-IIIof FIG. 1; and FIG. 4 is a cross-sectional view of the semiconductorlaser taken along line IV-IV of FIG. 1. It should be noted that in thesefigures, like numerals are used to denote like or correspondingcomponents.

Referring to FIG. 1, the semiconductor laser of the present embodimentis a waveguide ridge type DFB-LD 10 having an oscillation wavelength of1.3 μm and a resonator length L of approximately 100-250 μm, forexample.

A waveguide ridge 12 at the center portion of the DFB-LD 10 extends inthe laser light emission direction. Two electrode pad bases 16 aredisposed on respective sides of the waveguide ridge 12. A separationgroove 14 is disposed between each electrode pad base 16 and thewaveguide ridge 12.

An insulating film, for example, an SiO₂ film 18 is disposed on theentire surface of the DFB-LD 10 other than the surfaces of an opening 17provided at the center of the top portion of the waveguide ridge 12. Theopening 17 is formed at the center of the top portion of the waveguideridge 12 such that it extends in the axial direction of the resonator,or the x-direction.

Further, on the portion of the SiO₂ film 18 on the waveguide ridge 12 isdisposed a 1 μm thick metal film 19 formed of, for example, a Ti layer,a Pt layer, and an Au layer. At the top portion of the waveguide ridge12, the metal film 19 is in contact with a contact layer 22 or the toplayer of the waveguide ridge 12 through the opening 17 as a p-electrode20. A portion of the metal film 19 extends onto the electrode pad bases16 on both sides of the waveguide ridge 12. The above extended portionon the SiO₂ film 18 on each electrode pad base 16 constitutes a bondingpad 24 for connecting between the p-electrode 20 and an external wire(not shown).

In FIG. 1, arrow A represents laser light, and the direction of arrow Ais the emission direction of the light. In FIG. 1, reference numeral 25a denotes the emitting end face of the DFB-LD 10 through which laserlight is emitted, and reference numeral 25 b denotes the rear end faceon the side opposite to the emitting end face 25 a.

Referring now to FIG. 2, in the DFB-LD 10, an n-type cladding layer 28is disposed on a substrate 26, and a diffraction grating layer 30 isdisposed on the cladding layer 28. The substrate 26 is formed of n-typeInP and has a carrier concentration of 5×10¹⁸ cm⁻³ and a thickness of100 μm; the cladding layer 28 is formed of n-type InP and has a carrierconcentration of 1×10¹⁸ cm⁻³ and a thickness of 0.3 μm; and thediffraction grating layer 30 is formed of n-type InGaAsP and has acarrier concentration of 1×10¹⁸ cm⁻³ and a thickness of 0.05 μm.(n-type, p-type, and i-type (undoped) are hereinafter abbreviated as“n-”, “p-”, and “i-”, respectively.)

The diffraction grating layer 30 includes slits constituting adiffraction grating. An n-InP diffraction grating burying layer 32having a carrier concentration of 1×10¹⁸ cm⁻³ and a thickness of 0.1 μmis formed so as to fill these slits and cover the surface of thediffraction grating layer 30. On the diffraction grating burying layer32 is formed an n-AlGaInAs n-side optical confinement layer 34 having acarrier concentration of 1×10¹⁸ cm⁻³ and a thickness of 0.1 μm.

An undoped i-AlGaInAs active layer 36 having a thickness of 0.1 μm isdisposed on the n-side optical confinement layer 34. According to thepresent embodiment, the active layer 36 has a quantum well structure,and the barrier layer is formed of i-AlGaInAs. However, the active layer36 may be a bulk active layer.

A p-AlGaInAs p-side optical confinement layer 38 having a carrierconcentration of 1×10¹⁸ cm⁻³ and a thickness of 0.1 μm is disposed onthe active layer 36.

Further, a p-AlGaInAs first p-cladding layer 40 having a carrierconcentration of 0.5×10¹⁸ cm⁻³ and a thickness of 0.1 μm is disposed onthe p-side optical confinement layer 38.

A second p-cladding layer 42 is formed at the center portion of thewidth (in the y-direction) of the DFB-LD 10 such that it extends on thefirst p-cladding layer 40 from the emitting end face 25 a to the rearend face 25 b in the axial direction of the resonator. Further, thecontact layer 22 is disposed on the second p-cladding layer 42, and theyconstitute the waveguide ridge 12. The second p-cladding layer 42 isformed of p-InP and has a carrier concentration of 1×10¹⁸ cm⁻³ and athickness of 2.5 μm, while the contact layer 22 is formed of p-InGaAsand has a carrier concentration of 10×10¹⁸ cm⁻³ and a thickness of 0.5μm.

According to the present embodiment, the width of the waveguide ridge 12is larger in a specific portion above the phase shift region of thediffracting grating layer 30 than at the emitting end face 25 a and therear end face 25 b, as described later. It should be noted that thephase shift region of the diffraction grating layer 30 is located at thecenter portion of the length in the axial direction of the resonator andincludes a phase shift structure (denoted by reference numeral 30 b inFIG. 3).

The surface of the waveguide ridge 12 is covered with the 0.2 μm thickSiO₂ film 18, and the opening 17 is formed in the SiO₂ film 18 on thetop portion of the waveguide ridge 12. The p-electrode 20 is in contactwith and electrically connected to the contact layer 22 through theopening 17.

A 1 μm thick metal film formed of, for example, a Ti layer, a Pt layer,and an Au layer is disposed on the underside of the substrate 26 as ann-electrode 44.

In FIG. 3, arrow A represents the laser light, and the direction ofarrow A is the emission direction of the light. The diffraction gratinglayer 30 includes slits 30 a formed at predetermined intervals in theaxial direction of the resonator or the x-axis direction. These slits 30a are filled with the diffraction grating burying layer 32, so adiffracting grating being formed. Circle B in the figure indicates thephase shift structure 30 b located at the center portion of the lengthin the axial direction of the resonator.

FIG. 5 is a schematic diagram showing cross-sectional dimensions of thediffracting grating of the semiconductor laser according to oneembodiment of the present invention.

Referring to FIG. 5, in the DFB-LD 10 having an oscillation wavelengthof 1.3 μm, the dimensions of each regular grating element and each slit30 a of the diffraction grating layer 30 in the axial direction of theresonator or the x-axis direction are 0.1 μm. However, the dimension ofthe grating element of the phase shift structure of the diffractiongrating layer 30 in the axial direction of the resonator is 0.2 μm.

Although, in the DFB-LD 10, the diffraction grating layer 30 and thediffraction grating burying layer 32 are disposed between the n-claddinglayer 28 and the n-side optical confinement layer 34 so as to form adiffraction grating, a diffraction grating may be formed between thep-side optical confinement layer 38 and the first p-cladding layer 40.

FIG. 4 is a cross-sectional view of the DFB-LD 10, specifically, thesecond p-cladding layer 42 taken along a plane parallel to the layers ofthe waveguide ridge 12, that is, parallel to the main surface of thesubstrate 26, showing how the width of the waveguide ridge 12 varies inthe axial direction of the resonator.

The portion or the center portion of the waveguide ridge 12 that has alarger width than the other portions is located above or corresponds tothe phase shift region of the diffraction grating layer 30. This portionof the waveguide ridge 12 includes or constitutes a parallel portion 12a having a width of W1. This parallel portion 12 a is defined betweenthe two broken lines in FIG. 4.

The width of the waveguide ridge 12 gradually decreases from one end ofthe parallel portion 12 a toward the emitting end face 25 a and from theother end of the parallel portion 12 a toward the rear end face 25 b.The width of the waveguide ridge 12 at the emitting end face 25 a isdenoted by W2, and the width of the waveguide ridge 12 at the rear endface is denoted by W3. In the DFB-LD 10 of the present embodiment, thewidth W1 is 1.8 μm and the widths W2 and W3 are both 1.7 μm.

With the DFB-LD 10 having an oscillation wavelength of 1.3 μm, themaximum allowable value of the width W1 of the portion of the waveguideridge 12 located above or corresponding to the phase shift region isapproximately 1.8 μm, and the minimum allowable values of the widths W2and W3 of the waveguide ridge 12 at the emitting end face 25 a and therear end face 25 b, respectively, are both approximately 1.6 μm.

Thus, the width W1 is set to approximately 1.8 μm or less in order toprevent formation of high-order modes, and the widths W2 and W3 are setto approximately 1.6 μm or more to prevent leakage of light.

These values are only valid when the DFB-LD 10 has an oscillationwavelength of 1.3 μm. If the DFB-LD 10 has a different oscillationwavelength, the maximum and minimum allowable values are determinedbased on that oscillation wavelength in the same manner as describedabove.

FIG. 6 is a schematic diagram illustrating the photon densitydistribution within the resonator of the semiconductor laser of oneembodiment of the present invention, showing the relationship betweenthe position within the resonator and the photon density.

In FIG. 6, the vertical axis represents the photon density in arbitraryunits (a. u.), and the horizontal axis represents the position withinthe resonator also expressed in arbitrary units (a. u.). Further, symbolL indicates the resonator length.

FIG. 7 is a schematic diagram illustrating the optical confinementfactor distribution within the resonator of the semiconductor laser ofone embodiment of the present invention, showing the relationshipbetween the position within the resonator and the optical confinementfactor. In FIG. 7, the vertical axis represents the optical confinementfactor in arbitrary units (a. u.), and the horizontal axis representsthe position within the resonator also expressed in arbitrary units (a.u.). Further, symbol L indicates the resonator length.

FIG. 8 is a schematic diagram illustrating the relaxation oscillationfrequency distribution within the resonator of the semiconductor laserof one embodiment of the present invention, showing the relationshipbetween the position within the resonator and the relaxation oscillationfrequency. In the figure, the vertical axis represents the relaxationoscillation frequency in arbitrary units (a. u.), and the horizontalaxis represents the position within the resonator also expressed inarbitrary units (a. u.). Further, symbol L denotes the resonator length.In FIG. 8, curve a indicates the relaxation oscillation frequencydistribution of the DFB-LD 10, that is, the semiconductor laser of thepresent embodiment, and curve b indicates the relaxation oscillationfrequency distribution of a conventional semiconductor laser in whichthe width of the waveguide ridge does not vary in the axial direction ofthe resonator, a comparative example.

As shown in FIG. 6, the photon density distribution within the resonatorof the DFB-LD 10 is such that the photon density S gradually increasesfrom the emitting end face 25 a and the rear end face 25 b toward thephase shift region, dramatically increases in a region near the phaseshift structure 3 b, and then reaches its maximum value in the phaseshift structure 3 b. This photon density distribution tendency does notchange very much either the width of the waveguide ridge is varied inthe axial direction of the resonator or not.

In the DFB-LD 10, the width W1 of the portion of the waveguide ridge 12located above or corresponding to the phase shift region of thediffraction grating layer 30 is larger than those of the other portions.The width of the waveguide ridge 12 gradually decreases from thisportion toward the emitting end face 25 a and the rear end face 25 b.The widths of the waveguide ridge 12 at the emitting end face 25 a andat the rear end face 25 b are W2 and W3, respectively.

In the case of a waveguide ridge type DFB-LD, the optical confinementfactor G can generally be increased by reducing the ridge width. In theDFB-LD 10, the portion of the waveguide ridge 12 located above orcorresponding to the phase shift region of the diffraction grating layer30 has a large width W1, and the width of the waveguide ridge 12gradually decreases from this portion toward the emitting end face 25 ahaving the width W2 and the rear end face 25 b having the width W3.Therefore, as shown in FIG. 7, the optical confinement factordistribution within the resonator is such that the optical confinementfactor is high or maximized at the emitting end face 25 a and the rearend face 25 b, and gradually decreases from these end faces toward thephase shift region, dramatically decreases in a region near the phaseshift region, and then gradually decreases to its minimum value in theparallel portion 12 a having the width W1.

On the other hand, the relaxation oscillation frequency fr of asemiconductor laser, which represents the high-speed responsecharacteristics of the laser, is proportional to the square root of theproduct of the photon density S and the optical confinement factor G.

Therefore, in the DFB-LD 10, the relaxation oscillation frequency frdoes not vary much with the position within the resonator, as shown inFIG. 8, since a change in the photon density S is cancelled out by achange in the optical confinement factor G.

Specifically, the relaxation oscillation frequencies fr at the emittingend face 25 a and the rear end face 25 b are increased and the maximumrelaxation oscillation frequency fr is reduced, as compared to theconventional semiconductor laser, although the relaxation oscillationfrequency still rapidly increases in the phase shift region to somedegree.

As a result, the change in the relaxation oscillation frequency in theaxial direction of the resonator is reduced, making it possible toreduce the distortion of the modulated light waveform due to a change inthe relaxation oscillation frequency or due to non-uniform relaxationoscillation frequency distribution within the resonator when the DFB-LD10 is subjected to direct modulation operation at high speed.

FIG. 9 is a cross-sectional view of a variation of the semiconductorlaser according to one embodiment of the present invention.

Whereas in the DFB-LD 10 shown in FIG. 4 the width of the waveguideridge 12 is gradually changed, in the DFB-LD shown in FIG. 9 the widthof the waveguide ridge 12 is changed stepwise. This can also producesubstantially the same effects as described above.

As described above, the waveguide ridge type DFB-LDs 10 according to thepresent embodiment are configured such that: the second p-cladding layer42 forms the waveguide ridge 12; and the portion of this waveguide ridgelocated above or corresponding to the phase shift structure 30 b of thediffraction grating layer 30 provided at the center portion of thelength of the resonator has a larger width than the end portions of thewaveguide ridge. With this arrangement, the change in the photon densitywithin the resonator in the axial direction due to the phase shiftstructure 30 b is cancelled out by the change in the optical confinementfactor within the resonator in the axial direction due to a change inthe waveguide ridge width, a change in the relaxation oscillationfrequency within the resonator in the axial direction being reduced as aresult. Therefore, when the waveguide ridge type DFB-LD 10 is subjectedto direct modulation operation at high speed, it is possible to reducethe distortion of the modulated light waveform due to a change in therelaxation oscillation frequency or due to non-uniform relaxationoscillation frequency distribution within the resonator.

Second Embodiment

FIG. 10 is a perspective view of a semiconductor laser according to oneembodiment of the present invention; FIG. 11 is a cross-sectional viewof the semiconductor laser taken along line XI-XI of FIG. 10; FIG. 12 isa cross-sectional view of the semiconductor laser taken along lineXII-XII of FIG. 10; and FIG. 13 is a cross-sectional view of thesemiconductor laser taken along line XIII-XIII of FIG. 10.

Referring to FIG. 10, the semiconductor laser of the second embodimentof the present invention is a buried heterostructure DFB-LD 50 having anoscillation wavelength of 1.3 μm and a resonator length L ofapproximately 100-250 μm, for example.

A buried heterostructure 52 at the center portion of the DFB-LD 50extends in the laser light emission direction. In FIG. 10, arrow Arepresents the laser light, and the direction of arrow A is the emissiondirection of the light. Two electrode pad bases 16 are disposed onrespective sides of the buried heterostructure 52. A separate groove 14is disposed between each electrode pad base 16 and the buriedheterostructure 52. An insulating film, for example, an SiO₂ film 18 isdisposed on the entire surface of the DFB-LD 50 other than the surfacesof an opening 17 provided at the center of the top portion of the buriedheterostructure 52. The opening 17 is formed at the center of the topportion of the buried heterostructure 52 such that it extends in theaxial direction of the resonator, or the x-axis direction.

A metal film 19 is formed on the portion of the SiO₂ film 18 on theburied heterostructure 52. At the top portion of the buriedheterostructure 52, the metal film 19 is in contact with a contact layer22 or the top layer of the buried heterostructure 52 through the opening17 as a p-electrode 20. A portion of the metal film 19 extends onto theelectrode pad bases 16 on both sides of the buried heterostructure 52.The above extended portion on the SiO₂ film 18 on each electrode padbase 16 constitutes a bonding pad 24 for the p-electrode 20.

In FIG. 10, reference numeral 25 a denotes the emitting end face of theDFB-LD 50 through which the laser light indicated by arrow A is emitted,and reference numeral 25 b denotes the rear end face on the sideopposite to the emitting end face 25 a.

Referring now to FIG. 11, in the DFB-LD 50, an n-cladding layer 28 isdisposed on a substrate 26. The substrate 26 is formed of n-type InP andhas a carrier concentration of 5×10¹⁸ cm⁻³ and a thickness of 100 μm;and the n-cladding layer 28 is formed of n-InP and has a carrierconcentration of 1×10¹⁸ cm⁻³ and a thickness of 2 μm.

The buried heterostructure 52 is formed at the center portion of thewidth (in the y-direction) of the DFB-LD 50 such that it extends on then-cladding layer 28 from the emitting end face 25 a to the rear end face25 b in the axial direction of the resonator.

The buried heterostructure 52 includes: a mesa 54 located at the center;a burying portion 56 disposed on both sides of the mesa 54 such that themesa 54 is buried in the burying portion 56; a second p-cladding layer58 b disposed on the mesa 54 and the burying portion 56; and a contactlayer 22 disposed on the second p-cladding layer 58 b.

The mesa 54 includes: an n-InGaAsP diffraction grating layer 30 disposedon the n-cladding layer 28 and having a carrier concentration of 1×10¹⁸cm⁻³ and a thickness of 0.02 μm; an n-InP diffraction grating buryinglayer 32 filling and covering the diffraction grating layer 30 andhaving a carrier concentration of 1×10¹⁸ cm⁻³ and a thickness of 0.2 μm;an n-AlGaInAs n-side optical confinement layer 34 disposed on thediffraction grating burying layer 32 and having a carrier concentrationof 1×10¹⁸ cm⁻³ and a thickness of 0.1 μm, an undoped i-AlGaInAs activelayer 36 disposed on the n-side optical confinement layer 34 and havinga thickness of 0.1 μm; a p-AlGaInAs p-side optical confinement layer 38disposed on the active layer 36 and having a carrier concentration of1×10¹⁸ cm⁻³ and a thickness of 0.1 μm; and a p-InP first p-claddinglayer 58 a disposed on the p-side optical confinement layer 38 andhaving a carrier concentration of 1×10¹⁸ cm⁻³ and a thickness of 0.5 μm.

The burying portion 56 includes: a p-InP burying layer 60 disposed incontact with the sides of the mesa 54 and having a carrier concentrationof 2×10¹⁸ cm⁻³ and a thickness of 0.5 μm at the sides of the mesa 54;and an n-InP current blocking layer 62 defining the thickness of thep-InP burying layer 60 at the sides of the mesa 54 and having a carrierconcentration of 5×10¹⁸ cm⁻³ and a thickness of 2 μm.

The second p-cladding layer 58 b is formed of the same material as thefirst p-cladding layer 58 a and has a thickness of 2 μm, and the contactlayer 22 is formed of p-InGaAs and has a carrier concentration of10×10¹⁸ cm⁻³ and a thickness of 0.5 μm.

According to the second embodiment, the width of the mesa 54 is smallerin a specific portion located above or corresponding to the phase shiftregion of the diffraction grating layer 30 than at the emitting end face25 a and the rear end face 25 b, as described later. The phase shiftregion is located at the center portion of the length of the resonatorand includes a phase shift structure (denoted by reference numeral 30 bin FIG. 12).

In the etching process for forming the mesa 54, a selective etching maskpattern may be formed which has a width varying in the axial directionof the resonator.

The surface of the buried heterostructure 52 is covered with the SiO₂film 18, and the opening 17 is formed in the SiO₂ film 18 on the topportion of the buried heterostructure 52. The p-electrode 20 is incontact with and electrically connected to the contact layer 22 throughthe opening 17. An n-electrode 44 is disposed on the underside of thesubstrate 26.

In FIG. 12, arrow A represents the laser light, and the direction ofarrow A is the emission direction of the light. The diffraction gratinglayer 30 includes slits 30 a formed at predetermined intervals in theaxial direction of the resonator or the x-axis direction. These slits 30a are filled with the diffraction grating burying layer 32, forming adiffraction grating. In the figure, circle B indicates the phase shiftstructure 30 b located at the center portion of the length in the axialdirection of the resonator. The diffracting grating has the samecross-sectional dimensions as the diffraction grating of the firstembodiment described with reference to FIG. 5.

FIG. 13 is a cross-sectional view of the DFB-LD 50, specifically, theactive layer 36 taken along a plane parallel to the layers of the buriedheterostructure 52, that is, parallel to the main surface of thesubstrate 26, showing how the width of the mesa 54 varies in the axialdirection of the resonator.

The portion or the center portion of the mesa 54 that has a smallerwidth than the other portions is located above or corresponds to thephase shift region of the diffraction grating layer 30. This portion ofthe mesa 54 includes or constitutes a parallel portion 12 a having awidth of W1. This parallel portion 12 a is defined between the twobroken lines in FIG. 13.

The width of the mesa 54 gradually increases from one end of parallelportion 12 a toward the emitting end face 25 and from the other end ofthe parallel portion 12 a toward the rear end face 25 b. The widths ofthe mesa 54 at the emitting end face 25 a and at the rear end face 25 bare W2 and W3, respectively. In the DFB-LD 50 of the present embodiment,the width W1 is 0.4 μm and the widths W2 and W3 are both 1.5 μm.

With the DFB-LD 50 having an oscillation wavelength of 1.3 μm, theminimum allowable value of the width W1 of the portion of the mesa 54located above or corresponding to the phase shift region isapproximately 0.4 μm, and the maximum allowable values of the widths W2and W3 of the mesa 54 at the emitting end face 25 a and the rear endface 25 b, respectively, are both approximately 1.8 μm.

Thus, the width W1 is set to approximately 0.4 μm or more in order toprevent oscillation and facilitate manufacture of the device, and thewidths W2 and W3 are set to approximately 1.8 μm or less to preventformation of high-order modes.

These values are only valid when the DFB-LD 50 has an oscillationwavelength of 1.3 μm. If the DFB-LD 50 has a different oscillationwavelength, the maximum and minimum allowable values are determinedbased on that oscillation wavelength in the same manner as the firstembodiment.

As shown in FIG. 6 described in connection with the first embodiment,the photon density distribution within the resonator of the DFB-LD 50 issuch that the photon density S gradually increases from the emitting endface 25 a and the rear end face 25 b toward the phase shift region,dramatically increases in a region near the phase shift structure 3 b,and then reaches its maximum value in the phase shift structure 3 b.

In the DFB-LD 50, the width W1 of the portion of the mesa 54 locatedabove or corresponding to the phase shift region of the diffractiongrating layer 30 is smaller than the widths of the other portions.Specifically, the width of the mesa 54 gradually increases from thisportion toward the emitting end face 25 a and the rear end face 25 b.The widths of the mesa 54 at the emitting end face 25 a and at the rearend face 52 b are W2 and W3, respectively.

In the case of a buried heterostructure DFB-LD, the optical confinementfactor G can generally be increased by increasing the ridge width. Inthe DFB-LD 50, the portion of the mesa 54 located above or correspondingto the phase shift region of the diffraction grating layer 30 has asmall width (W1), and the width of the mesa 54 gradually increases fromthis portion toward the emitting end face 25 a having the width W2 andthe rear end face 25 b having the width W3. Therefore, as shown in FIG.7 described in connection with the first embodiment, the opticalconfinement factor distribution within the resonator is such that theoptical confinement factor G is high or maximized at the emitting endface 25 a and the rear end face 25 b, and gradually decreases from theseend faces toward the phase shift region, dramatically decreases in aregion near the phase shift region, and then gradually decreases to itsminimum value in the parallel portion 12 a having the width W1.

On the other hand, the relaxation oscillation frequency fr of asemiconductor laser, which represents the high-speed responsecharacteristics of the laser, is proportional to the square root of theproduct of the photon density S and the optical confinement factor G.

Therefore, in the DFB-LD 50, the relaxation oscillation frequency frdoes not vary much with the position within the resonator, as shown inFIG. 8 described in connection with the first embodiment, since a changein the photon density S is cancelled out by a change in the opticalconfinement factor G.

Specifically, the relaxation oscillation frequencies fr at the emittingend face 25 a and the rear end face 25 b are increased and the maximumrelaxation oscillation frequency fr is reduced, as compared to aconventional semiconductor laser, although the relaxation oscillationfrequency still rapidly increases in the phase shift region to somedegree. As a result, the change in the relaxation oscillation frequencyin the axial direction of the resonator is reduced, making it possibleto reduce the distortion of the modulated light waveform due to a changein the relaxation oscillation frequency or due to non-uniform relaxationoscillation frequency distribution within the resonator when the DFB-LD50 is subjected to direct modulation operation at high speed.

FIG. 14 is a cross-sectional view of a variation of the semiconductorlaser according to one embodiment of the present invention.

Whereas in the DFB-LD 50 shown in FIG. 13 the width of the mesa 54 isgradually changed, in the DFB-LD 50 shown in FIG. 14 the width of themesa 54 is changed stepwise. This can also produce substantially thesame effects as described above.

As described above, the buried heterostructure DFB-LDs 50 of the secondembodiment are configured such that: a heterojunction is formed in themesa 54 including the active layer 36; the mesa 54 is buried in theburying portion 56 disposed on both sides of the mesa 54; and theportion of the mesa 54 located above or corresponding to the phase shiftstructure 30 b of the diffraction grating layer 30 provided at thecenter portion of the length of the resonator has a smaller width thanthe end portions of the mesa 54. With this arrangement, the change inthe photon density within the resonator in the axial direction due tothe phase shift structure 30 b is cancelled out by the change in theoptical confinement factor within the resonator in the axial directiondue to a change in the mesa width, a change in the relaxationoscillation frequency within the resonator in the axial direction beingreduced as a result. Therefore, when the buried heterostructure DFB-LDis subjected to direct modulation operation at high speed, it ispossible to reduce the distortion of the modulated light waveform due toa change in the relaxation oscillation frequency or due to non-uniformrelaxation oscillation frequency distribution within the resonator.

As described above, a semiconductor laser according to the presentinvention comprises: a semiconductor substrate of a first conductivetype; a first cladding layer of the first conductive type located on thesemiconductor substrate; an active layer located on the first claddinglayer; a second cladding layer of a second conductive type located onthe active layer; and a diffraction grating layer located between theactive layer and the first or second cladding layer and including aphase shift structure which is formed in a part of the diffractiongrating layer in an optical waveguide direction, wherein an opticalconfinement factor is reduced in corresponding to the phase shiftstructure of the diffraction grating layer. This arrangement allows theproduct of the photon density and the optical confinement factor in thespecific portion in corresponding to the phase shift structure to besubstantially equal to that in the other portions, resulting in therelaxation oscillation frequency in the specific portion beingsubstantially equal to the relaxation oscillation frequency in the otherportions. This means that the relaxation oscillation frequency does notvary much in the axial direction of the resonator, preventing distortionof the modulated light waveform and allowing the semiconductor laser toachieve good high-speed response characteristics.

Third Embodiment

FIG. 15 is a cross-sectional view of a semiconductor laser according toone embodiment of the present invention.

The semiconductor laser shown in FIG. 15 is a waveguide ridge typeDFB-LD 70 similar to the DFB-LD 10 of the first embodiment.Specifically, the DFB-LD 70 has the same configuration as the DFB-LD 10except that the diffraction grating layer 30 of the DFB-LD 70 does notinclude the phase shift structure 30 b. Therefore, a cross-sectionalview of the DFB-LD 70 taken along a plane perpendicular to the resonatoraxis is similar to the cross-sectional view shown in FIG. 2.

Further, the cross-sectional view of the DFB-LD 70, specifically, thesecond p-cladding layer 42 shown in FIG. 15 corresponds to that shown inFIG. 4 and is taken along a plane parallel to the layers of thewaveguide ridge 12, that is, parallel to the main surface of thesubstrate 26, showing how the width of the waveguide ridge 12 vary inthe axial direction of the resonator.

In the DFB-LD 70, the diffraction grating layer 30 does not include thephase shift structure 30 b, and the emitting end face 25 a has a lowerreflectance than the rear end face 25 b. As a result, the photon densitydistribution within the resonator of the DFB-LD 70 is such that thephoton density gradually decreases from the emitting end face 25 a tothe rear end face 25 b. Therefore, the DFB-LD 70 is further configuredsuch that: the width of the waveguide ridge 12 is largest at theemitting end face 52 a and smallest at the rear end face 52 b; and thewidth of the waveguide ridge 12 linearly decreases from the emitting endface 52 a to the rear end face 52 b. This causes an optical confinementfactor distribution to be formed within the resonator such that theoptical confinement factor G is highest at the rear end face 25 b andgradually decreases toward the emitting end face 25 a.

On the other hand, the relaxation oscillation frequency fr of asemiconductor laser, which represents the high-speed responsecharacteristics of the laser, is proportional to the square root of theproduct of the photon density S and the optical confinement factor G.Therefore, the relaxation oscillation frequency distribution within theresonator of the DFB-LD 70 is such that the relaxation oscillationfrequency does not vary much with the position within the resonator,since a change in the photon density S is cancelled out by a change inthe optical confinement factor G.

Specifically, the relaxation oscillation frequency fr is increased atthe rear end face 25 b and decreased at the emitting end face 25 a ascompared to when the waveguide ridge has a constant width, thus reducingthe change in the relaxation oscillation frequency in the axialdirection of the resonator. Therefore, when the DFB-LD 70 is subjectedto direct modulation operation at high speed, it is possible to reducethe distortion of the modulated light waveform due to a change in therelaxation oscillation frequency or due to non-uniform relaxationoscillation frequency distribution within the resonator.

As described above, a semiconductor laser according to the presentembodiment comprises: a semiconductor substrate of a first conductivetype; a first cladding layer of the first conductive type located on thesemiconductor substrate; an active layer located on the first claddinglayer; a second cladding layer of a second conductive type located onthe active layer; a diffraction grating layer located between the activelayer and the first or second cladding layer; and an optical waveguideridge including the a second cladding layer, wherein

the waveguide ridge is larger in the width at one end having a higherphoton density in an optical waveguide direction than at the other end.This arrangement reduces the change in the relaxation oscillationfrequency within the resonator in the axial direction. Therefore, whenthe semiconductor laser is subjected to direct modulation operation athigh speed, it is possible to reduce the distortion of the modulatedlight waveform due to a change in the relaxation oscillation frequencyor due to non-uniform relaxation oscillation frequency distributionwithin the resonator.

Fourth Embodiment

FIG. 16 is a cross-sectional view of a semiconductor laser according toa fourth embodiment of the present invention.

The semiconductor laser shown in FIG. 16 is a buried heterostructureDFB-LD 75 similar to the DFB-LD 50 of the second embodiment.Specifically, the DFB-LD 75 has the same configuration as the DFB-LD 50except that the diffraction grating layer 30 of the DFB-LD 75 does notinclude the phase shift structure 30 b. Therefore, a perspective view ofthe DFB-LD 75 is similar to the perspective view shown in FIG. 10described in connection with the second embodiment. Further, across-sectional view of the DFB-LD 75 taken along a plane perpendicularto the resonator axis is similar to the cross-sectional view shown inFIG. 11 described in connection with the second embodiment.

Further, the cross-sectional view of the DFB-LD 75, specifically, theactive layer 36 shown in FIG. 16 corresponds to that shown in FIG. 13and is taken along a plane parallel to the layers of the buriedheterostructure 52, that is, parallel to the main surface of thesubstrate 26, showing how the width of the mesa 54 varies in the axialdirection of the resonator.

In the DFB-LD 75, the diffraction grating layer 30 does not include thephase shift structure 30 b, and the emitting end face 25 a has a lowerreflectance than the rear end face 25 b. As a result, the photon densitydistribution within the resonator of the DFB-LD 75 is such that thephoton density gradually decreases from the emitting end face 25 a tothe rear end face 25 b.

Therefore, the DFB-LD 75 is further configured such that: the width ofthe mesa 54 is smallest at the emitting face 52 a and largest at therear end face 52 b; and the width of the mesa 54 linearly decreases fromthe rear end face 25 b to the emitting end face 52 a. This causes anoptical confinement factor distribution to be formed within theresonator such that the optical confinement factor G is highest at therear end face 25 b and gradually decreases toward the emitting end face25 a.

On the other hand, the relaxation oscillation frequency fr of asemiconductor laser, which represents the high-speed responsecharacteristics of the laser, is proportional to the square root of theproduct of the photon density S and the optical confinement factor G.Therefore, the relaxation oscillation frequency distribution within theresonator of the DFB-LD 75 is such that the relaxation oscillationfrequency does not vary much with the position within the resonator,since a change in the photon density S is cancelled out by a change inthe optical confinement factor.

Specifically, the relaxation oscillation frequency fr is increased atthe rear end face 25 b and decreased at the emitting end face 25 a ascompared to when the mesa has a constant width, thus reducing the changein the relaxation oscillation frequency in the axial direction of theresonator. Therefore, when the DFB-LD 75 is subjected to directmodulation operation at high speed, it is possible to reduce thedistortion of the modulated light waveform due to a change in therelaxation oscillation frequency or due to non-uniform relaxationoscillation frequency distribution within the resonator.

In the semiconductor laser of each embodiment described above, the widthof a waveguide ridge or a mesa including an active layer and aheterojunction is varied to vary the optical confinement factoraccording to the position within the resonator in the axial direction.However, the thickness of at least one of the layers that are disposedbetween an n-side cladding layer and a p-side cladding layer and thathave a higher refractive index than the n-side or p-side cladding layer,that is, an active layer, a barrier layer, optical confinement layers,etc. may be varied in the optical waveguide direction such that it isreduced in a specific portion adjacent or corresponding to a relativelyhigh photon density region. This can also produce the same effects asdescribed above.

Reducing the thickness of these high refractive index layers results ina reduction in the optical confinement factor, making it possible toprevent the relaxation oscillation frequency from varying in the axialdirection of the resonator. Therefore, when the DFB-LD 75 is subjectedto direct modulation operation at high speed, it is possible to reducethe distortion of the modulated light waveform due to a change in therelaxation oscillation frequency or due to non-uniform relaxationoscillation frequency distribution within the resonator.

The thicknesses of the above high refractive index layers may be variedby an MOCVD device using a selective growth technique.

Fifth Embodiment

FIG. 17 is a schematic diagram showing a pitch of a diffraction gratingaccording to one embodiment of the present invention.

FIG. 18 is a schematic diagram showing the relationship between theposition within a resonator and the equivalent refractive indexaccording to one embodiment of the present invention.

FIG. 19 is a schematic diagram showing the diffraction grating pitchdistribution within the resonator according to one embodiment of thepresent invention.

This embodiment is applied to the DFB-LDs of the first and secondembodiments. The oscillation wavelength λ of each DFB-LD is proportionalto the product of the diffraction grating pitch D and the equivalentrefractive index n_(eff), and, generally, the equivalent refractiveindex n_(eff) decreases with decreasing optical confinement factor ofthe active layer.

As shown in FIG. 18, in these DFB-LDs in which the diffraction gratingincludes a phase shift structure, the equivalent refractive indexdramatically decreases to its minimum value in the phase shift region.

Since the oscillation wavelength λ is proportional to the product of thediffraction grating pitch D and the equivalent refractive index n_(eff),a large change in the equivalent refractive index n_(eff) with theposition within the resonator results in a change in the oscillationwavelength λ.

Therefore, in the DFB-LD according to the present embodiment, the pitchD of the diffraction grating is not fixed, but varied with the positionwithin the resonator such that the product of the pitch D and theequivalent refractive index n_(eff) is substantially constant with theposition within the resonator. That is, if the equivalent refractiveindex n_(eff) varies with the position within the resonator as shown inFIG. 18, the diffraction grating of the DFB-LD is formed such that thegrating pitch varies with the position within the resonator as shown inFIG. 19.

This allows the DFB-LD to emit laser light with a narrow spectral widthpredominantly including the desired oscillation wavelength.

A manufacturing method for varying the pitch of the diffraction gratingaccording to the position within the resonator in the axial direction isto use EB (Electron Beam) lithography to form a diffraction gratingforming pattern used in a conventional semiconductor laser.

As described above, a semiconductor laser according to the presentinvention comprises: a semiconductor substrate of a first conductivetype; a first cladding layer of the first conductive type located on thesemiconductor substrate; an active layer located on the first claddinglayer; a second cladding layer of a second conductive type located onthe active layer; and a diffraction grating layer located between theactive layer and said first or second cladding layer, wherein the pitchof the diffraction grating layer is varied in an axial direction of theresonator such that the product of the pitch and an equivalentrefractive index is substantially constant in the axial direction, theequivalent refractive index being determined by the refractive index ofeach layer. This arrangement reduces a variation in the oscillationwavelength λ, allowing the semiconductor laser to emit laser light witha narrow spectral width predominantly including the desired oscillationwavelength.

It should be noted that although the above embodiments have beendescribed as using AlGaInAs as a material for the active layer, othermaterials may be used, such as InGaAsP, InGaAs, InGaP, InGaNAs, andInGaAsSb.

As described above, the semiconductor lasers according to the presentinvention are especially suitable for use as light sources for opticalfiber communications.

While the presently preferred embodiments of the present invention havebeen shown and described. It is to be understood these disclosures arefor the purpose of illustration and that various changes andmodifications may be made without departing from the scope of theinvention as set forth in the appended claims.

1. A semiconductor laser comprising: a semiconductor substrate of afirst conductivity type; a first cladding layer of the firstconductivity type, supported by said semiconductor substrate; an activelayer supported by said first cladding layer; a second cladding layer ofa second conductivity type, supported by said active layer; adiffraction grating layer located between said active layer and one ofsaid first and second cladding layers and including a phase shiftstructure in a central part of said semiconductor laser, with respect toan optical waveguide direction, of said semiconductor laser; and a layerhaving a higher refractive index than that of said first or secondcladding layers, located between said first and second cladding layers,and reduced in thickness at said phase shift structure.
 2. Asemiconductor laser comprising: a semiconductor substrate of a firstconductivity type; a first cladding layer of the first conductivitytype, supported by said semiconductor substrate; an active layersupported by said first cladding layer; a second cladding layer of asecond conductivity type, supported by said active layer; and adiffraction grating layer located between said active layer and one ofsaid first and second cladding layers and including a phase shiftstructure in a central part of said semiconductor laser, with respect toan optical waveguide direction of said semiconductor laser, wherein saidsemiconductor laser includes a waveguide ridge including at least partof said second cladding layer and said waveguide ridge is wider in awidth direction, transverse to the optical waveguide direction, at saidphase shift structure than at least one of first and second ends,transverse to the optical waveguide direction, of said waveguide ridge.3. A semiconductor laser comprising: a semiconductor substrate of afirst conductivity type; a first cladding layer of the firstconductivity type, supported by said semiconductor substrate; an activelayer supported by said first cladding layer; a second cladding layer ofa second conductivity type, supported by said active layer; and adiffraction grating layer located between said active layer and one ofsaid first and second cladding layers and including a phase shiftstructure in a central part of said semiconductor laser, with respect toan optical waveguide direction of said semiconductor laser, wherein saidsemiconductor laser includes a mesa-shaped heterostructure extending inthe optical waveguide direction and including at least parts of saidactive layer and said first and second cladding layers, and a buryinglayer burying said mesa-shaped heterostructure, wherein said mesa-shapedheterostructure is narrower in a width direction, transverse to theoptical waveguide direction, at said phase shift structure than at leastone of first and second ends, transverse to the optical waveguidedirection, of said waveguide ridge.
 4. The semiconductor laser accordingto claim 2, wherein said waveguide ridge is wider at said phase shiftregion than at said first end of said waveguide ridge, and saidwaveguide ridge changes gradually in width from said phase shiftstructure toward said first end of said waveguide ridge.
 5. Thesemiconductor laser according to claim 2, wherein said waveguide ridgeis wider at said phase shift region than at said first end of saidwaveguide ridge, and said waveguide ridge changes in steps in width fromsaid phase shift structure toward said first end of said waveguideridge.
 6. The semiconductor laser according to claim 2, wherein saidwaveguide ridge is wider at said phase shift region than at both of saidfirst and second ends of said waveguide ridge, and said waveguide ridgechanges gradually in width from said phase shift structure toward bothof said first and second ends of said waveguide ridge.
 7. Thesemiconductor laser according to claim 2, wherein said waveguide ridgeis wider at said phase shift region than at both of said first andsecond ends of said waveguide ridge, and said waveguide ridge changes insteps in width from said phase shift structure toward both of said firstand second ends of said waveguide ridge.
 8. The semiconductor laseraccording to claim 3, wherein said waveguide ridge is narrower at saidphase shift region than at said first end of said waveguide ridge, andsaid waveguide ridge changes gradually in width from said phase shiftstructure toward said first end of said waveguide ridge.
 9. Thesemiconductor laser according to claim 3, wherein said waveguide ridgeis narrower at said phase shift region than at said first end of saidwaveguide ridge, and said waveguide ridge changes in steps in width fromsaid phase shift structure toward said first end of said waveguideridge.
 10. The semiconductor laser according to claim 3, wherein saidwaveguide ridge is narrower at said phase shift region than at both ofsaid first and second ends of said waveguide ridge, and said waveguideridge changes gradually in width from said phase shift structure towardboth of said first and second ends of said waveguide ridge.
 11. Thesemiconductor laser according to claim 3, wherein said waveguide ridgeis narrower at said phase shift region than at both of said first andsecond ends of said waveguide ridge, and said waveguide ridge changes insteps in width from said phase shift structure toward both of said firstand second ends of said waveguide ridge.